Various forms of detonation diamond-containing materials, produced by an explosion of carbon-containing explosive supplies, are known. For example, in order to produce a detonation diamond-containing material, which material may correspond to various mixtures of nanodiamonds and non-diamond forms of carbon, explosives with negative oxygen balance, containing trinitrotoluol, (TNT) and cyclotrimethylenetrinitramine (RDX), are utilized. The products obtainable by detonation synthesis may comprise a diamond-containing detonation blend (DB) and detonation nanodiamonds (DND), wherein the latter may be extracted upon chemical purification of DB from non-diamond carbon and metal-containing impurities.
Diamond-containing detonation blend physically corresponds to a complex mix of nanodiamonds and non-diamond carbon forms, wherein DB particles generally comprise a diamond core of 4-6 nm, surrounded by X-ray amorphous carbon. In accordance with research studies (Dolmatov V. Y., Detonation synthesis ultradispersion diamonds, S-Petersburg, 2003, RU2230702), closest to a diamond core non-diamond carbon corresponds to agglomerated ultrasmall particles of distorted diamond phase nuclei, which particles did transform to diamond because of exhaustion of free carbon. Overall amount of diamond core surrounding non-diamond carbon atoms is in average 50-70% from the overall amount of carbon atoms of DB-particle. Outer boundaries of such a particle correspond to a non-diamond carbon with an uppermost degree of disorder and of a highest reactivity. For this reason outer non-diamond carbon layer of DB particles is so easily oxidized even by weak oxidants.
Detonation nanodiamonds may be in turn extracted from the above mentioned DB by means of DB treatment by aqueous solutions of nitric acid under pressure. In accordance to RU2100389, DND purification process proceeds in liquid phase in two stages, wherein the first stage comprises a treatment of DB by aqueous solutions of 50-99% nitric acid at a temperature range 80-180° C., followed by a second stage DB treatment by aqueous solutions of 10-40% nitric acid at about 250° C. Resulting DND particles comprise a 4-6 nm diamond core, surrounded on its periphery by a very thin layer (about 0.4-0.1 nm) of the most resistant to oxidation non-diamond carbon. Upon producing DND particles from DB, virtually all non-diamond carbon is gasified during the process of chemical purification. Research (Dolmatov V. Y., Detonation Nanodiamonds. Production, properties, application. S-Petersburg-Professional, 2011) has shown, that above mentioned DND particles correspond to complex objects, retaining a three-layer structure, said structure comprises a diamond core of 4-6 nm diameter, comprising 70-90% of overall amount of carbon atoms contained in the particle; a transitional carbon shell surrounding the core, 0.4-0.1 nm thick and comprising non-diamond (X-ray amorphous) carbon, that is most resistant to oxidation and may comprise 30-10% of overall amount of carbon atoms contained in the particle; and a surface layer, comprising carbon atoms and nitrogen, oxygen and hydrogen heteroatoms, included into original explosive composition, which atoms produce various functional groups. However, non-diamond carbon forms neither separate phase nor separate particles. Diamond and non-diamond carbon forms differentiate by electronic states of atoms and chemical reactivity in liquid phase oxidants. High quality DND may comprise 0.4-1.5 wt. % of non-diamond carbon.
Nevertheless, the main problem, preventing effective utilization of DB and DND in various application technologies, is presence of metal-containing impurities therein. Heavy metals, in particular iron, cause detonation nanodiamond particles to agglomerate, increases already existing agglomerates in size and degrades the stability of nanodiamond suspensions (Aleksensky et al, Nanoscience and Nanotechnology Letters (3):68-74, 2011). Although many applications of detonation nanodiamonds in chemistry of polymers, superfinish polishing, in oils and lubricants, electroplating, medicine and biology, require a substantial degree of purification from major disturbing impurities represented by incombustible, water-insoluble metal-containing compounds, the content of metal contaminants in most of the commercially available products is still too high. Those metal-containing impurities most commonly act as undesirable catalysts or inhibitors of key processes, wherein only detonation nanodiamond-containing materials are expected to work. Incombustible impurities are primarily represented by oxides of those metals that compose blasting chamber casing, detonator conducting wires and detonator itself.
Common tryouts for processes of chemical DB purification and DND after-purification are targeted on getting rid of non-diamond carbon and metal-containing impurities at once by utilizing mainly liquid-phase oxidants, that are capable of providing high concentrations of the reagents in reaction zone, and correspondingly, of ensuring high reaction speed (RU2230702; RU2100389; Dolmatov V. Y., Detonation Nanodiamonds. Production, properties, application. S-Petersburg-Professional, 2011). Because liquid-phase oxidation at a reasonable speed occurs at high temperature, oxidative mixtures are being prepared from high-boiling acids, such as HClO4, H3PO4, H2SO4 with an addition of active oxidant, such as HNO3, NaClO4, CrO3, K2CrO7 and the like. Utilization of HClO4, however, is dangerous and expensive. It is of common knowledge (e.g. RU2230702), that chromium trioxide (CrO3) in sulfuric acid is utilized most often in DND production. Purification process thus comprises treatment of DND by boiling sulfuric acid for few hours. The process is simple, but toxic because of presence of large amount of Cr+6 both in solution and in extracted DND aggregates. Chromium outflow, resulting from multiple washings of DND from acid and chromium traces, is also hazardous. Utilization of an aqueous solution of nitric acid successfully solves several problems with outflow and waste, however the process is complicated since to be conducted at high temperature (about 230° C.) and pressure (up to 100 atmospheres) and requires special equipment.
Methods for the selective purification of DB from non-diamond carbon and/or metal-containing impurities are also known. Thus, in RU2019502 non-diamond carbon purification of diamond-containing detonation blend is achieved by oxidation of DB by ozone, and in RU2168462—by heating of DB in air atmosphere at 380-440° C. The method of RU2019502 is quite expensive and dangerously explosive, and implementation of method in accordance with RU2168462 is risky because of possible ignition of DB and DND, contained therein. At the same time those methods do not solve the key problem of nanodiamond product's purification from metal-containing impurities; therefore a treatment with harsh oxidative mixtures is still required. Thus, DND purified by the most general ‘chromium’ method still comprise an essential amount of incombustible water-insoluble metal-containing impurities, including CrO3, Cr2O3 and other chromium oxides from a number of CrO, Cr3O4, CrO2, Cr5O12, Cr2O5, Cr6O15 and Cr3O8. Impurities, most widely occurring in DB, comprise FeO, Fe3O4, CuO, Cu2O, ZnO and small amounts of MnO and NiO. Virtually all these compounds are insoluble in water, and are partially removed upon treatment with acids and alkali. At present it is possible to get rid of 10-15% of metal-containing impurities and of 5-7% of a most active non-diamond carbon by treatment of DB by boiling concentrated nitric acid for 3-5 hours, in accordance with RU2046094.
Utilization of DB in chemistry of polymers and as an additive for oils and lubricants requires substantially complete removal of metal-containing impurities while not affecting non-diamond carbon that plays an important role in various processes. Moreover, for the utilization of DND in medicine and biology it is of vital importance to get rid of poisonous chromium, copper and manganese. However, acidic or alkali treatment of certain nanodiamond-containing materials may sometimes not be acceptable for a number of reasons. For example, acidic treatment of DB will reduce an amount of incombustible (mainly metal-containing) impurities 3-4-fold, while the same treatment of previously partly purified DND is will have no effect at all, since DND particles aggregate after such a treatment into hardly destructible objects, and an access of the acid to encapsulated metal oxide particles is very limited. Utilization of aqueous solutions of alkali, such as KOH or NaOH, is dangerous because of DB and DND predisposition to ignite upon alkali treatment thereof at elevated temperatures.
It is therefore desirable to provide an efficient method to produce detonation nanodiamond materials with a substantially high degree of purity in regards to metal containing impurities, such as toxic Cr or agglomeration-inducing Fe, for example, which method may be applied as a post-purification procedure for any commercially available detonation nanodiamond material. It would be further desirable whether said method would be non-aggressive, less toxic and ecologically beneficial.